Method of assessing at least one petroleum characteristic of a rock sample

ABSTRACT

The invention is a Method of assessing at least one petroleum characteristic of a rock sample. Starting from a temperature ranging between 50° C. and 120° C., the temperature of a rock sample is raised to a temperature ranging between 180° C. and 220° C. which is maintained for a predetermined time duration. The temperature of the sample is increased to a temperature ranging between 330° C. and 370° C. which is maintained for a predetermined time duration. The temperature of the sample is then raised to a temperature ranging between 630° C. and 670° C. Three quantities representative of the amount of hydrocarbon compounds released during the temperature change stages are measured and at least one petroleum characteristic of the sample is deduced from these quantities.

CROSS REFERENCE TO RELATED APPLICATION

Reference is made to French Patent Application Serial No. 14/60.576,filed Nov. 3, 2014, and French Patent Application Serial No. 14/55.009,filed Jun. 3, 2014, which application is incorporated herein byreference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the petroleum industry and moreparticularly exploration and exploitation of a potentially or trulyhydrocarbon-producing geological formation.

2. Description of the Prior Art

Petroleum compounds, mainly hydrocarbons extractable using organicsolvents, form through the transformation of insoluble organic matter(known as kerogen) generated by the temperature and pressure riseinherent in the burial of sediments in a sedimentary basin. Sedimentaryrocks that have generated petroleum are referred to as source rocks.Under certain conditions, the petroleum thus formed is partly expelledfrom the source rock and migrates to rocks known as reservoir rockswhere it accumulates.

Source rocks have long been considered as an unexploitable source ofhydrocarbons. This is no longer the case with the evolution oftechnology. Indeed, hydrocarbons from source rocks represent a newsource of fossil energy today. For this type of hydrocarbon, the sourcerock is also the reservoir rock, the hydrocarbons remaining trapped anddisseminated within a very low-permeability rock. Source rockhydrocarbons are categorized in the family of unconventionalhydrocarbons, their extraction requiring unconventional techniques inrelation to those commonly used in the oil and gas industry. This typeof source rock is known as hydrocarbon source rock (shale plays).

Knowing the soluble organic matter (petroleum), on the one hand, and theinsoluble organic matter (kerogen), on the other hand, present in a rocksample is of great interest in oil exploration and exploitation. Indeed,it is for example well known that:

the amount of petroleum formed in the sediments regularly increases withthe depth of burial. It is therefore possible to assess the degree ofevolution of the organic matter contained in these sediments and, moreparticularly, the interval of evolution that corresponds to the mainstage of petroleum formation,

the nature of the insoluble organic matter contained in the rocksconditions the petroleum-generating potential of these rocks, that is,their more or less high capacity to produce petroleum.

Thus, knowledge of such data allows drilling to carry out long, costlyand perilous coring or reservoir testing operations only advisedly,especially when these results can be acquired simultaneously with thedrilling operations.

The following documents are mentioned in the description below:

Behar, F., Beaumont, V., De B. Penteado, H. L., 2001. Rock-Eval 6Technology: Performances and Developments. Oil & Gas Science andTechnology 56, 111-134.

Lafargue, E., Marquis, F., Pillot, D., 1998. Rock-Eval 6 Applications inHydrocarbon Exploration, Production, and Soil Contamination Studies. Oil& Gas Science and Technology—Revue de l′Institut Francais du Pétrole,53, 4, 421-437.

The known ROCK-EVAL® technique (IFP Energies nouvelles, France)developed by the applicant is notably described in French Patent No.2,472,754 and European Patent No. 0,691,540. This analysis technique,which is fast and almost automatic, has been developed forcharacterizing the organic matter and the hydrocarbons contained in arock sample from a geological formation. More precisely, this techniqueallows determination of the presence, the nature and the degree ofmaturity of the organic matter contained in a rock sample. The ROCK-EVALmethod also provides precise information on the quantification ofhydrocarbons, the amount of total organic carbon (TOC) and the amount ofmineral carbon (MinC) contained in a rock sample.

The ROCK-EVAL technique involves the pyrolysis in an inert(non-oxidizing) atmosphere, according to a predetermined temperaturesequence, of a rock sample. The pyrolysis oven cooperates with a devicefor detecting and measuring the amount of hydrocarbon compounds in thepyrolyzed sample. The specific detection device comprises for example aflame ionization type detector conventionally used in gas chromatographyanalyses. The detector delivers a signal representative of the amountsof hydrocarbon products measured. This signal can be transmitted tocomputation, storage and display means wherein a specific softwarecomputes, displays and stores the various parameters representative ofthe characteristics of the hydrocarbons present. More details concerningthe ROCK-EVAL technique are given in the aforementioned documents.

Thus, the ROCK-EVAL technique allows in particular measuring the amountof hydrocarbon compounds released throughout the pyrolysis. A pyrogramcan then be established, which is a curve showing the evolution of theamount of released hydrocarbon compounds in relation to the weight ofthe sample being considered, as a function of time. A pyrogram generallyhas several peaks (see for example FIGS. 1A and 1B). The peaks are welldifferentiated and the surface area of each peak is calculated. Aquantity representative of the amount of hydrocarbon compounds releasedwithin the temperature range bounding the peak considered is thusobtained for each peak.

Two main methods using two different temperature sequences have beendeveloped:

The Basic or Bulk Rock method, more particularly dedicated to sourcerock samples, is for example described in Lafargue et al. (1998) andBehar et al. (2001). The temperature sequence of this method ischaracterized by an initial temperature T1 of the pyrolysis ovengenerally ranging between 300° C. and 350° C., a temperature that ismaintained for a predetermined time duration of the order of someminutes. It is during this stage that the free hydrocarbons initiallycontained in the rock sample are released. Their amount is estimated bymeasuring the surface area of a first peak, denoted by S₁. The pyrolysistemperature is then progressively raised to a temperature T2 ofgenerally 550° C. This stage involves the volatilization of the veryheavy hydrocarbon compounds and the cracking of the non-volatile organicmatter. The amount of hydrocarbon compounds released during this thermalcracking stage is assessed by measuring the surface area of a secondpeak, denoted by S₂. It corresponds to the amount of hydrocarboncompounds that would have been generated if the rock had reached asufficient maturation stage;

The Reservoir method, more particularly dedicated to reservoir rocksamples, is for example described in European Patent No. 0,691,540. Thetemperature sequence of the Reservoir method is characterized by aninitial temperature T1 of the pyrolysis oven below 200° C. andpreferably equal to 180° C. This temperature is maintained for apredetermined time duration and the amount of light hydrocarboncompounds is assessed by measuring the surface area of a first peak,denoted by S_(1r). The temperature of the pyrolysis oven is then raisedto a second temperature T2 of about 370° C., a stage wherein the amountof heavier hydrocarbons released is estimated by assessing the surfacearea of a second peak, denoted by S_(2a). Temperature T2 substantiallycorresponds to the end of the thermovaporization of some hydrocarbonsand to the start of cracking through pyrolysis of the heavy compounds.Thus, the family of hydrocarbon compounds corresponding to peaks S_(1r)and S_(2a) of the Reservoir method is nearly equivalent to the family ofhydrocarbon compounds characteristic of peak S₁ of the Basic method. Thepyrolysis temperature is then increased again to a third temperature T3of at most 800° C. The surface area of a third peak, denoted by S_(2b),representative of the heavy hydrocarbon compounds, is estimated duringthis third heating stage. This peak S_(2b) can be considered as anequivalent of peak S₂ of the Basic method.

Although initially developed for particular rock types (source rock forthe Basic method and reservoir rock for the Reservoir method), these twomethods can however be applied to any rock type. Applied to samples ofhydrocarbon source rock type, it appears that both methods underestimatethe surface area of the peak(s) corresponding to the free hydrocarbonscontained in the sample considered. Indeed, as illustrated in FIGS. 1Aand 1B showing the pyrograms resulting from the application of the Basicmethod and of the Reservoir method respectively to the same hydrocarbonsource rock sample, the first peak is truncated. The quantification ofthe surface area thereof is therefore underestimated, as well as theamount of free hydrocarbon compounds contained in this rock.

Thus, it appears that the temperature sequences of the Basic andReservoir methods are in particular not suitable for hydrocarbon sourcerock samples. This can be explained by the fact that, when these methodshave been developed, source rocks were not considered to be exploitablepetroleum reservoirs.

Due to the growing interest in source rock hydrocarbons, there is a highdemand for an improvement in the ROCK-EVAL pyrolysis technique allowingto reliably assess the amount of free hydrocarbons in a hydrocarbonsource rock, and more generally the amount of hydrocarbons potentiallyextractable from this type of rock.

SUMMARY OF THE INVENTION

The present invention relates to a pyrolysis method allowing improvementof the quantification of hydrocarbon compounds contained in a rocksample. In particular, the temperature sequence applied in the presentinvention is suited for hydrocarbon source rocks. Furthermore, somepetroleum characteristics allow evaluation of the interest of exploringand/or developing a given geological formation are predefined.

The invention relates to a method of assessing at least one petroleumcharacteristic of a rock sample from a geological formation, wherein thesample is heated in an inert atmosphere according to a temperaturesequence. The method comprises the following stages:

a) from a first temperature value (T1) ranging between 50° C. and 120°C., raising the temperature of the sample, according to a firsttemperature gradient ranging between 1° C./min and 50° C./min, to asecond temperature value (T2) ranging between 180° C. and 220° C., andmaintaining the sample at the second temperature value (T2) for a firstpredetermined time duration;

b) from the second temperature value (T2), raising the temperature ofthe sample according to a second temperature gradient ranging between 1°C./min and 50° C./min to a third temperature value (T3) ranging between330° C. and 370° C., and maintaining the sample at the third temperaturevalue (T3) for a second predetermined time duration;

c) from the third temperature value (T3), raising the temperature of thesample according to a third temperature gradient ranging between 1°C./min and 50° C./min to a fourth temperature value (T4) ranging between630° C. and 670° C.;

d) determining quantities S_(h0), S_(h1) and S_(h2) representative ofthe measured amount of hydrocarbon compounds released in stages a, b andc respectively;

e) determining, from at least one of the quantities S_(h0), S_(h1) andS_(h2), at least one petroleum characteristic of the sample.

In one embodiment of the present invention, the rock sample can comefrom a hydrocarbon source rock.

According to an advantageous embodiment, the first temperature can rangebetween 80° C. and 120° C.

According to another advantageous embodiment, the first temperature canrange between 90° C. and 110° C.

Advantageously, at the start of stage a), the sample can be maintainedat the first temperature T1.

According to a preferred embodiment, the second temperature can rangebetween 190° C. and 210° C.

According to another preferred embodiment, the third temperature canrange between 340° C. and 360° C.

Advantageously, the fourth temperature can range between 640° C. and660° C.

Preferably, the first and second time durations can range between 2 and4 minutes.

Advantageously, the first, second and third temperature gradients canrange between 20° C./min and 30° C./min.

According to an embodiment of the present invention, the petroleumcharacteristic of the sample calculated in stage e) can be selected fromamong: a free hydrocarbon content index denoted by HC_(cont), a qualityindex of the hydrocarbons denoted by HQI, a production index denoted byPI_(Shale), the API degree, the GOR parameter and the GCR parameter.

According to an embodiment of the present invention, a free hydrocarboncontent index HC_(cont) can be calculated in stage e) according to theformula as follows:

HC _(cont) =S _(h0) +S _(h1)

with S_(h0), S_(h1) and HC_(cont) expressed in milligram of hydrocarboncompound per gram of rock.

According to another embodiment of the present invention, a qualityindex HQI of the hydrocarbons can be calculated in stage e) according tothe formula as follows:

${HQI} = {\frac{S_{h\; 0}}{S_{h\; 0} + S_{h\; 1}} \times 100}$

with S_(h0) and S_(h1) expressed in milligram of hydrocarbon compoundper gram of rock and HQI expressed as percentage by mass.

According to a particular embodiment of the present invention, aproduction index PI_(Shale) can be calculated in stage e) according tothe formula as follows:

${PI}_{Shale} = {\frac{\left( {S_{h\; 0} + S_{h\; 1}} \right)}{\left( {S_{h\; 0} + S_{h\; 1} + S_{h\; 2}} \right)} \times 100}$

with S_(h0), S_(h1) and S_(h2) expressed in milligram of hydrocarboncompound per gram of rock, and PI_(Shale) expressed as percentage bymass.

Advantageously, the calculation of the production index PI_(Shale) canbe repeated for the samples from different sedimentary layers of theformation, and at least one sedimentary layer of the formation ofinterest with a view to oil exploration and/or exploitation can bedetermined when the production index PI_(Shale) is greater by a factor λthan the average of all of the production indices PI_(Shale) measuredfor the formation.

Preferably, said factor λ can range between 1.1 and 1.5.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the method according to the inventionwill be clear from reading the description hereafter of embodimentsgiven by way of non limitative example, with reference to theaccompanying figures wherein:

FIGS. 1A and 1B show the evolution of the amount of hydrocarboncompounds (Q) over time (t) during a pyrolysis, established according tothe Basic method and to the Reservoir method respectively for the samehydrocarbon source rock sample. It can be noted that the X-axisrepresenting time is given on an indicative basis: the position of thepeaks in relation to the X-axis cannot be directly compared because thetemperature sequences are different from one method to the other;

FIG. 2 illustrates the temperature sequence of the method according tothe invention;

FIG. 3 illustrates the evolution of the amount of hydrocarbon compounds(Q) over time (t) during a pyrolysis, established according to themethod of the invention on a hydrocarbon source rock sample, the samplebeing identical to the one used in FIGS. 1A and 1B. It can be noted thatthe X-axis representing time is given on an indicative basis: theposition of the peaks in relation to the X-axis cannot be directlycompared because the temperature sequences are different from one methodto the other.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is a method of assessing at least one petroleumcharacteristic of a rock sample from a geological formation, whereinsample is heated in an inert atmosphere according to a particulartemperature sequence.

The present invention is applicable to any rock type such as, forexample, a source rock, a reservoir rock or a hydrocarbon source rock.The advantages of the method according to the invention are shownhereafter within the context of an application to a rock sample from ahydrocarbon source rock.

Thus, the present invention comprises at least the following stages:

a) from a first temperature value (T1) ranging between 50° C. and 120°C., raising the temperature of the sample, according to a firsttemperature gradient ranging between 1° C./min and 50° C./min, to asecond temperature value (T2) ranging between 180° C. and 220° C., andmaintaining the sample at the second temperature value (T2) for a firstpredetermined time duration;

b) from the second temperature value (T2), raising the temperature ofthe sample according to a second temperature gradient ranging between 1°C./min and 50° C./min to a third temperature value (T3) ranging between330° C. and 370° C., and maintaining the sample at the third temperaturevalue (T3) for a second predetermined time duration;

c) from the third temperature value (T3), raising the temperature of thesample according to a third temperature gradient ranging between 1°C./min and 50° C./min to a fourth temperature value (T4) ranging between630° C. and 670° C.;

d) determining quantities S_(h0), S_(h1) and S_(h2) representative ofthe measured amount of hydrocarbon compounds released in stages a, b andc respectively;

e) determining, from at least one of the three quantities S_(h0), S_(h1)and S_(h2), at least one petroleum characteristic of the sample.

FIG. 2 illustrates the temperature sequence of the pyrolysis operationin an inert atmosphere according to the method of the invention.

At the beginning of the process, the sample is fed into the oven heatedat a first temperature (T1). This initial temperature value (T1) rangesbetween 50° C. and 120° C. According to a preferred embodiment of theinvention, first temperature (T1) ranges between 80° C. and 120° C.According to another embodiment of the present invention, firsttemperature (T1) ranges between 90° C. and 110° C.

According to an embodiment of the present invention, the sample ismaintained at first temperature (T1) for a non-zero time duration. Thispreliminary stage of maintaining the sample at first temperature (T1)allows warming up of the sample and/or release of the very lighthydrocarbon compounds present in a slightly damaged or undamaged sample.

The sample is then subjected to a programmed heating phase where thetemperature rise ranges between 1° C./min and 50° C./min (segment A), upto a temperature corresponding to a second temperature (T2) rangingbetween 180° C. and 220° C. Advantageously, a second temperature (T2)ranging between 190° C. and 210° C. is selected. The temperaturegradient preferably ranges between 20° C./min and 30° C./min.

The sample is maintained at second temperature (T2) for a firstpredetermined non-zero time duration (segment B) greater than ahalf-minute for example and preferably ranging between 2 and 4 minutes.This second temperature (T2) substantially corresponds to the end of thethermovaporization stage of the lighter hydrocarbons contained in therock sample and to the start of the stage of cracking through pyrolysisof the heavier hydrocarbons.

Then, from second temperature (T2), the temperature is raised (segmentC) up to a temperature corresponding to a third temperature (T3)according to a temperature gradient ranging between 1° C./min and 50°C./min. The value of third temperature (T3) ranges between 330° C. and370° C. Advantageously, a third temperature value between 340° C. and360° C. is selected. According to a preferred embodiment, thetemperature gradient ranges between 20° C./min and 30° C./min.

Third temperature (T3) is maintained (segment D) for a predeterminednon-zero time duration greater than a half-minute for example andpreferably ranging between 2 and 4 minutes. This third temperature (T3)substantially corresponds to the end of the thermovaporization stage ofthe heavy hydrocarbons contained in the rock sample and to the start ofthe stage of cracking through pyrolysis of the very heavy hydrocarbons.

Pyrolysis is continued (segment E) so as to reach a fourth temperature(T4) according to a temperature gradient ranging between 1° C./min and50° C./min. The value of fourth temperature (T4) ranges between 630° C.and 670° C., preferably between 640° C. and 660° C. The temperaturegradient advantageously ranges between 20° C./min and 30° C./min. Thisfourth temperature (T4) substantially corresponds to the end of thepyrolysis stage, that is, the end of the thermal cracking of the organicmatter present in the rock sample.

Thus, the temperature sequence of the method according to the inventioncomprises a succession of three heating stages (ramps illustrated bysegments A, C and E in FIG. 2) separated by two temperature maintenancestages (isothermal stages illustrated by segments B and D in FIG. 2).

FIG. 3 shows the pyrogram resulting from the application of the methodaccording to the invention to the same hydrocarbon source rock sample asthe one considered for establishing FIGS. 1A and 1B. It can be observedin FIG. 3 that the pyrogram is characterized by the presence of threepeaks: a first peak referred to as peak S_(h0), a second peak referredto as peak S_(h1) and a third peak referred to as peak S₂.

The surface area of peak S_(h0), given for example in milligram ofhydrocarbon compounds per gram of rock, corresponds to the amount ofhydrocarbon compounds obtained between first temperature (T1) and secondtemperature (T2), more precisely obtained during segments A and B ofFIG. 2. This quantity, referred to as quantity S_(h0), is representativeof the lightest thermovaporizable hydrocarbons.

The surface area of peak S_(h1), given for example in milligram of inhydrocarbon compounds per gram of rock, corresponds to the amount ofhydrocarbon compounds obtained between second temperature (T2) and thirdtemperature (T3), more precisely obtained during segments C and D ofFIG. 2. This quantity, referred to as quantity S_(h1), is representativeof the heavy thermovaporizable hydrocarbons.

The surface area of peak S_(h2), given for example in milligram ofhydrocarbon compounds per gram of rock, corresponds to the amount ofhydrocarbon compounds obtained between third temperature (T3) and fourthtemperature (T4), more precisely obtained during segment E of FIG. 2.This quantity, referred to as quantity S_(h2), is representative of thevery heavy thermovaporizable hydrocarbons.

It can be observed in FIG. 3 that the method according to the inventionallows obtaining a first complete, non-truncated peak (peak S_(h0)),unlike the Basic (peak S₁ in FIG. 1A) and Reservoir (peak S_(1r) in FIG.1B) methods. The method according to the invention thus allows moreexhaustive recording of the free hydrocarbons contained in a sample andtherefore, more generally, better quantification of the hydrocarboncompounds contained in a sample. Furthermore, the method according tothe invention allows better separation of the last peak (peak S_(h2))than in the case of the Reservoir method (peak S_(2b) in FIG. 1B) or inthe case of the Basic method (peak S₂ in FIG. 1A).

Thus, the advantages of the method according to the invention shown inthe above example are explained by the temperature sequencecharacteristic of the method according to the invention. Indeed, as thetemperature sequence starts at a first low temperature (T1) in relationto the prior art methods, the method according to the invention allowsmeasuring more completely the amount of free hydrocarbon compoundspresent in a sample. Furthermore, the method of the inventioncomprising, between two heating stages (ramps A, C and E in FIG. 2),temperature maintenance stages (isothermal stages B and D in FIG. 2) ofduration at least greater than one half-minute, these isothermal stagesallow ensuring the end of the thermovaporization of the hydrocarboncompounds of interest in the temperature range considered.

From at least one of these three quantities, at least one petroleumcharacteristic of the source rock sample considered is calculated.

According to an embodiment of the present invention, a free hydrocarboncontent index HC_(cont) is calculated according to the formula asfollows:

HC _(cont) =S _(h0) +S _(h1)

with S_(h0), S_(h1) and HC_(cont) expressed in milligram of hydrocarbonsper gram of rock.

According to another embodiment of the present invention, a qualityindex HQI of the hydrocarbons is calculated according to the formula asfollows:

${HQI} = {\frac{S_{h\; 0}}{S_{h\; 0} + S_{h\; 1}} \times 100}$

with S_(h0) and S_(h1) expressed in milligram per gram of rock. HQIrepresents the proportion, expressed as percentage by mass, of verylight hydrocarbons in relation to the thermovaporizable fraction.

Advantageously, a production index PI_(Shale) is calculated according tothe formula as follows:

${PI}_{Shale} = {\frac{\left( {S_{h\; 0} + S_{h\; 1}} \right)}{\left( {S_{h\; 0} + S_{h\; 1} + S_{h\; 2}} \right)} \times 100}$

with S_(h0), S_(h1) and S_(h2) expressed in milligram per gram of rock.Index PI_(Shale), expressed as percentage by mass, represents thequantity relative to the light hydrocarbon fraction in relation to allof the pyrolyzable hydrocarbons (thermovaporizable hydrocarbons plusthose starting to be thermocracked).

According to an embodiment of the present invention wherein theassessment of index PI_(Shale) is repeated for rock samples fromdifferent sedimentary layers of a geological formation, the sedimentarylayer(s) of the geological formation of interest with a view to oilexploration and/or exploitation are determined when their productionindex PI_(Shale) is greater by a factor λ than the average of all theproduction indices PI_(Shale) measured for the geological formationconsidered. According to a preferred embodiment of the invention, avalue ranging between 1.1 and 1.5 is selected for factor λ.

According to an embodiment of the present invention, it is also possibleto calculate, from said three quantities, a petroleum characteristic ofthe source rock sample considered, such as:

The API degree: it is a scale known to the person skilled in the art,which is a measure of the gravity of a crude oil; thus, the lighter acrude oil, the lower the gravity and the higher the API degree thereof.Most crudes have API degrees ranging between 20° (very heavy) and 60°(very light);

The GOR (Gas/Oil Ratio) parameter: this measure is known to the personskilled in the art expresses the amount of gas produced at the wellheadin relation to the amount of oil;

The GCR (Gas/Condensate Ratio) parameter: this measure is known to theperson skilled in the art expresses the amount of condensates (or lighthydrocarbons) in relation to the amount of gas.

APPLICATION EXAMPLES

Table 1 compares the results obtained with the method according to theinvention (referred to as Method 1) and the Reservoir method (referredto as Method 2) on three rock samples (referred to as sample A, B and C)from different hydrocarbon source rock types. In the case of theReservoir method, equivalences denoted by HC*_(cont), HQI* andPI*_(Shale) for petroleum characteristics HC_(cont), HQI and PI_(Shale)defined above for the method according to the invention were calculatedas follows:

${{{{{{{- {HC}_{{cont}\;}^{*}} = {S_{1\; r} + S_{2a}}};} - {HQI}^{*}} = {\frac{S_{1r}}{S_{1r} + S_{2a}} \times 100}};} - {PI}_{Shale}^{*}} = {\frac{\left( {S_{1r} + S_{2a}} \right)}{\left( {S_{1r} + S_{2a} + S_{2b}} \right)} \times 100.}$

According to this table, it can be observed that, whatever the sampleunder consideration, the value of the petroleum characteristic HC_(cont)obtained with the method according to the invention is greater than itsequivalent HC*_(cont) obtained with the Reservoir method. Indeed, asshown in Table 1, characteristic HC_(cont) is 30% greater than itsequivalent HC*_(cont) in the case of sample A, 40% in the case of sampleB and 28% in the case of sample C. Thus, this means that the methodaccording to the invention allows measuring a larger amount of freehydrocarbons contained in the sample considered. As a result, by meansof a temperature sequence starting with a lower temperature than theReservoir method, the method according to the invention allows betterassessment of the amount of the free hydrocarbons present in a rocksample from a hydrocarbon source rock than the prior art.

As regards petroleum characteristics HQI and PI_(Shale), no systematictrend can be observed from one sample to the next. This is explained bythe fact that these characteristics depend on the proportion of thehydrocarbon types (free, thermovaporizable, thermocrackable) present inthe sample considered respectively.

Table 2 compares the measured quantities S_(h0), S_(h1) and S_(h2)(corresponding to the surface areas of peaks S_(h0), S_(h1) and S_(h2)respectively), as well as some petroleum characteristics (HC_(cont), HQIand PI_(Shale)) obtained with the method of the invention applied to arock sample containing source rock hydrocarbons, implemented with thefollowing values for the first, second, third and fourth temperatures:

-   -   case 1: T1=100° C., T2=200° C., T3=350° C. and T4=650° C.;    -   case 2: T1=80° C., T2=200° C., T3=350° C. and T4=650° C.;    -   case 3: T1=100° C., T2=180° C., T3=350° C. and T4=650° C.;    -   case 4: T1=100° C., T2=220° C., T3=350° C. and T4=650° C.

It can be seen in Table 2 that petroleum characteristic HC_(cont) isequivalent (to within 1.7%, which is the order of magnitude of themeasurement uncertainty) in cases 1 or 2. This shows that the freehydrocarbons contained in a rock sample are recovered in an equivalentmanner when applying the method according to the invention either withthe central temperature or with the minimum temperature of the preferredfirst temperature range relative to first temperature T1.

It can also be observed in Table 2 that measured quantity S_(h0) variesgreatly in cases 3 and 4. Thus, the value of second temperature T2,corresponding to the end of the first heating ramp and to the firsttemperature stage of the temperature sequence implemented in the presentinvention, has a significant impact on quantity S_(h0). Indeed, at 180°C., all the light free hydrocarbons have not been thermovaporized yet(S_(h0) is 1.59 mg/g rock) whereas at 220° C., thermovaporization ofpart of the heavier free hydrocarbons has started (S_(h0) is 2.5 mg/grock). On the other hand, once again, the value of petroleumcharacteristic HC_(cont) is equivalent in cases 3 and 4 (to within 0.01mg/g rock, which is below the measurement uncertainty). This shows thatthe free hydrocarbons contained in a rock sample are recovered in anequivalent manner when applying the method according to the inventioneither with the minimum temperature or with the maximum temperature ofthe temperature range relative to second temperature T2.

TABLE 1 S_(h0) S_(h1) S_(h2) HC_(cont) HQI PI_(Shale) Method 1 (mg/g)(mg/g) (mg/g) (mg/g) (mass %) (mass %) Sample A 1.72 3.03 2.39 4.7536.21 66.53 Sample B 5.03 7.50 96.04 12.53 40.14 11.54 Sample C 0.912.75 6.33 3.66 24.86 36.64 S_(1r) S_(2a) S_(2b) HC*_(cont) HQI* HQI*Method 2 (mg/g) (mg/g) (mg/g) (mg/g) (mass %) (mass %) Sample A 1.252.04 1.25 3.29 37.99 72.47 Sample B 3.33 4.29 95.70 7.62 43.70 7.38Sample C 0.70 1.93 6.97 2.63 26.62 27.40

TABLE 2 Sh0 Sh1 Sh2 HCcont HQI PIShale (mg/g) (mg/g) (mg/g) (mg/g) (mass%) (mass %) Case 1 1.94 2.85 2.51 4.79 40.50 65.62 Case 2 1.89 2.82 2.444.71 40.13 65.87 Case 3 1.59 3.31 2.54 4.90 32.45 65.86 Case 4 2.50 2.412.53 4.91 50.92 65.99

Other Embodiments

According to a particular embodiment of the present invention, a rocksample from a geological formation is heated in a non-oxidizingatmosphere with the temperature sequence as defined in the methodaccording to the invention, and the amount of hydrocarbon compoundsreleased during the heating stage is continuously measured using a firstdetector, as well as the amount of CO₂ and of CO contained in theeffluent resulting from the heating stage, using a second detector.According to one embodiment of the present invention, from thecontinuous CO₂ measurements, the amount of CO₂ of organic origin and theamount of CO₂ of mineral origin is determined. According to anotherembodiment of the present invention, from the continuous CO₂ and COmeasurements, the amount of oxygen of organic origin and the amount ofoxygen of mineral origin is determined. According to another embodimentof the present invention, the residues resulting from heating in anon-oxidizing atmosphere are placed in another oven where they areheated in an oxidizing atmosphere. According to an embodiment of thepresent invention, heating in an oxidizing atmosphere can be temperatureprogrammed so as to raise the temperature from approximately 400° C. toapproximately 850° C. with a temperature gradient ranging between 10 and30° C./min. According to an embodiment of the present invention, theamount of CO₂ and/or of CO contained in the effluent resulting fromheating in an oxidizing atmosphere is continuously measured. Accordingto an embodiment of the present invention, from the continuous CO₂measurements in an oxidizing atmosphere, the amount of CO₂ of organicorigin and the amount of CO₂ of mineral origin is determined. Accordingto another embodiment of the present invention, the amount of totalorganic carbon contained in the sample is determined from the CO₂ and COmeasurements obtained after heating sequences in non-oxidizing andoxidizing atmospheres. According to an embodiment of the presentinvention, the device used for implementing the present inventioncomprises a single oven allowing heating in an oxidizing atmosphere andheating in a non-oxidizing atmosphere.

17. A method of assessing at least one petroleum characteristic of arock sample from a geological formation, wherein a sample is heated inan inert atmosphere according to a temperature sequence, comprising: a)raising a temperature of the sample from a first temperature rangingbetween 50° C. and 120° C., according to a first temperature gradientranging between 1° C./min and 50° C./min, to a second temperatureranging between 180° C. and 220° C., and maintaining the sample at thesecond temperature for a first predetermined time duration; b) raisingthe temperature of the sample from the second temperature according to asecond temperature gradient ranging between 1° C./min and 50° C./min toa third temperature ranging between 330° C. and 370° C., and maintainingthe sample at the third temperature for a second predetermined timeduration; c) raising the temperature of the sample from the thirdtemperature according to a third temperature gradient ranging between 1°C./min and 50° C./min to a fourth temperature ranging between 630° C.and 670° C.; d) determining three quantities representative of themeasured amount of hydrocarbon compounds released in steps a), b) and c)respectively; e) determining, from at least one of the three quantities,at least one petroleum characteristic of the sample.
 18. A method asclaimed in claim 17, wherein the rock sample comprises a hydrocarbonsource rock.
 19. A method as claimed in claim 17, wherein the firsttemperature ranges between 80° C. and 120° C.
 20. A method as claimed inclaim 17, wherein the first temperature ranges between 90° C. and 110°C.
 21. A method as claimed in claim 19, wherein the first temperatureranges between 90° C. and 110° C.
 22. A method as claimed in claim 17wherein, at a start of step a), the sample is maintained at the firsttemperature.
 23. A method as claimed in claim 17, wherein the secondtemperature ranges between 190° C. and 210° C.
 24. A method as claimedin claim 19, wherein the second temperature ranges between 190° C. and210° C.
 25. A method as claimed in claim 17, wherein the thirdtemperature ranges between 340° C. and 360° C.
 26. A method as claimedin claim 19, wherein the third temperature ranges between 340° C. and360° C.
 27. A method as claimed in claim 17, wherein the fourthtemperature ranges between 640° C. and 660° C.
 28. A method as claimedin claim 19, wherein the fourth temperature ranges between 640° C. and660° C.
 29. A method as claimed in claim 25, wherein the fourthtemperature ranges between 640° C. and 660° C.
 30. A method as claimedin claim 18, wherein the first and second time durations range between 2and 4 minutes.
 31. A method as claimed in claim 17, wherein the first,second and third temperature gradients range between 20° C./min and 30°C./min.
 32. A method as claimed in claim 17, wherein the petroleumcharacteristic of the sample calculated in e) is selected from a freehydrocarbon content index denoted by a quality index of hydrocarbons, aproduction index, an API degree, a GOR parameter and a GCR parameter.33. A method as claimed in claim 32, wherein the free hydrocarboncontent index is calculated according to a formula as follows:HC _(cont) =S _(h0) +S _(h1) wherein S_(h0), S_(h1) and HC_(cont) areexpressed in milligrams of hydrocarbon compound per gram of rock.
 34. Amethod as claimed in claim 32, wherein a quality index HQI of thehydrocarbons is calculated according to a formula as follows:${HQI} = {\frac{S_{h\; 0}}{S_{h\; 0} + S_{h\; 1}} \times 100}$wherein S_(h0) and S_(h1) are expressed in milligrams of hydrocarboncompound per gram of rock and HQI is expressed as percentage by mass.35. A method as claimed in claim 32, wherein a production index iscalculated according to the formula as follows:${PI}_{Shale} = {\frac{\left( {S_{h\; 0} + S_{h\; 1}} \right)}{\left( {S_{h\; 0} + S_{h\; 1} + S_{h\; 2}} \right)} \times 100}$wherein S_(h0), S_(h1) and S_(h2) an expressed in milligrams ofhydrocarbon compound per gram of rock, and PI_(Shale) is expressed as apercentage by mass.
 36. A method as claimed in claim 35, whereincalculation of the production index is repeated for the samples fromdifferent sedimentary layers of the formation, and at least onesedimentary layer of the formation being considered for explorationand/or exploitation is determined when the production index is greaterby a factor λ than the average of all of the production indicesPI_(Shale) measured for the formation.
 37. A method as claimed in claim36, wherein the factor λ ranges between 1.1 and 1.5.